Transmission Efficiency of plastic Films Part 1

TRANSMISSION EFFICIENCY OF PLASTIC FILMS
When correct dispersion has been achieved the blend is removed via a valve at the
base of the mixing chamber and weighed out into batch tins. If pigment is required it
is added at this stage to each separate batch. The batch tins are then emptied into a
primary mixer, such as a Ban bury, and treated for about five to ten minutes at 120-
160°C. The combination of heat and the ‘kneading’ action of the Ban bury causes
partial gelatin of the mix. This partly gelled material is then passed to a two-roll mill
and made to form a sheet round the front roll. This may be fed directly to the calendar
but for thin sheet or film a straining process is added to remove any coarse particles.
A typical strainer consists of a screw extruder with a filter screen directly in front of
the screw. The screen consists of a fine stainless steel woven wire mesh backed up
by a medium to coarse mesh for mechanical support, together with a perforated
strainer plate.
Calendars may consist of from two to five hollow rolls (or bowls) arranged for steam
heating or water cooling and are characterized by the number of bowls their
arrangement as for example, I, Z or inverted L types. Four roll, inverted L and Z types
are the most usual.
Very high forces have to be exerted on the rolls in order to squeeze the plastics to a
thin film and these cause the rolls to bend giving sheet that is thicker in the middle
than at the edges.
Z types have the advantage in this respect because bending of the rolls has no effect
on the succeeding nip and so improves thickness accuracy. This is because
alternative rolls are at right angles rather than in-line.
As mentioned earlier, thickness of the film or sheet leaving the calendar is measured
using a beta-ray gauge. This gives an average reading of thickness over an area of
sheet. Radiation from an isotope such as Thallium 204 is passed through the sheet
and then collected in an ionization chamber. The radiation received by the chamber
is inversely proportional to the weight per unit area of the material being measured.
Advantage of calendaring is that better mixing is obtained. The amount of energy
available in a calendaring line is very much more than in an extrusion line, so
calendared film is less dependent on the uniformity of the feedstock.
The main advantage of extrusion for PVC is the much lower capita costs involved, so
that shorter runs can be made at economic rates.
2.6 SOLVENT CASTING:
Solvent casting methods are expensive but cellulose nitrate film was made in this
way because of the flammable nature of the film. A casting solution is prepared by
dissolving cellulose nitrate and camphor (as plasticizer) in a 70:30 ether/alcohol
mixture using paddle mixers. After 8—10 h mixing the product is filtered through
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